Answers for "Sound" notes page

1. What is vibrating in these sound-makers?

(a) brass instruments (such as the tuba, trumpet and trombone)  your lips
(b) clarinet and saxophone  the reed
(c) drum  the drumhead

2. Measure the wavelength of the sound wave drawn with the tuning forks to the left. What is the frequency of this sound wave? Can humans hear it?

On my screen, the distance from one compression to the next is 1.5 cm. (Your screen might be a little different.) Using the speed of sound of 343 m/sec, that results in a frequency of 23 kHz, higher than what humans can hear.

3. What is the range of sound wavelengths detectable by the ears of

(a) a dog     0.0068m (6.8mm) to 23m
(b) a bat      0.0028m (2.8mm) to 0.34m
(c) a human 0.017m (1.7cm) to 17m

4. Sound travels at about 1470 m/sec in seawater. A note of 512 Hz has what wavelength in (a) air, and (b) seawater?   (a) 0.670m in air    (b) 2.87m in seawater

5. In steel, sound travels at about 5050 m/sec. Will a sound in steel will have a shorter, longer, or the same wavelength as the same-frequency sound in air?   Longer. Look at the equation. For a given frequency, a higher velocity means a higher wavelength.

6. The speed of sound in air varies a little, depending on the temperature of the air. Thinking about what sound is, do you think the speed of sound increases or decreases with rising air temperature? Increases. Sound compressions move when atoms and molecules in the air bump into their neighbors, compressing them. The faster they are moving (i.e. the hotter they are) the faster they can do that.

7. Why do you think sound travels faster in solids than in air?  Solids are denser than air, so the atoms and molecules must be packed closer together. The closer the particles are, the faster the compression can move.

8. Submarines use sonar to detect surface ships and other submarines. A sound wave (a 'ping') is sent out in all directions, which hits the other ship and reflects (echos) back. The direction of the echo and the time delay give the location of the target. If the time delay between emitting the sound and hearing the echo is 1.7 seconds, how far away is the target? Use 1470 m/sec as the speed of sound (see question #4). The target is 1250 meters away. (Don't forget to divide by 2, because the sound makes a round trip.)

9. If a tree falls in the forest, and no one is there to hear it, does it make a sound?  This old philosophical question really isn't as tricky as it seems. It all hinges on how you define the word sound. When the tree falls, it will create compressions and rarefactions in the air that will travel outward at approximately 343 m/sec. If you call those longitudinal waves "sound", as we have in this class, then yes, the tree makes sound. Some people, though, define "sound" not as the waves themselves, but our perception of those waves. If that's your definition, then if no one hears the waves there is no sound.

10. How many times louder is:

(a) the rustle of leaves than the threshold of hearing? 10 dB, or 10 times louder
(b) busy traffic than a whisper?  50 dB, or 100,000 times louder